In 1999, the newspaper proprietor Conrad Black offered Boris Johnson the editorship of the Spectator on the condition that he abandon his plans to become a Conservative MP. Boris readily agreed. He later said of his twin careers in journalism and politics, “The horses are starting to get further apart and the straddling operation is becoming increasingly stressful on the, um, crotch region.”

Two years later, when Boris was elected the MP for Henley, Conrad Black would have been within his rights to sack him. Instead, he threw a party in his employee’s honour, inviting the cream of London society to celebrate “the Boris phenomenon”. He even commissioned a composer to write a song about the blonde bombshell – and the musician was none too happy when, at the end of the evening, Boris purloined the sheet music for his personal archive.

Not everyone let down by Boris is quite so quick to forgive and Sonia Purnell, an ex-Daily Telegraph journalist, is one such victim. She was given the unenviable task of working as his “number two” when he was the paper’s man in Brussels and he was loathe to throw her even the smallest bone on account of his pathological competitiveness. On one memorable occasion, Boris was so angry about losing a point to his sister in a game of table tennis – or “whiff whaff”, as he calls it – that he kicked the table and broke his toe.

Revenge is a dish best served cold and sixteen years later Purnell has produced a muckraking biography. She dredges up the usual litany of embarrassing stories – the occasion when he was contacted by the fraudster Darius Guppy wanting the home address of an investigative reporter that was on his trail, his forced departure from the Times when he was caught making up a quote, his numerous extra-marital affairs – and generally does all she can to persuade the reader that he’s a thoroughly bad egg. “I think he is the most ruthless, ambitious person I have ever met,” she told Marina Wheeler, Boris’s wife, in 1991 and she evidently hasn’t changed her mind.

The fact that Purnell has so little affection for her chosen subject must have made writing this 450-page biography a bit of a chore. There are several pages of sources, all meticulously recorded in tiny print, and a further six pages of notes, faithfully reproducing the titles of all the books and articles she’s read by and about the Johnson clan. It must have been quite an undertaking, given that Boris’s father Stanley, as well as his sister Rachel and brother Jo, are all prolific scribblers. Yet what should have been an enjoyable romp, piecing together the colourful life of one of the most flamboyant characters in British public life, was a joyless task.

There’s plenty of interesting material here. Purnell reveals that Stanley methodically set about the task of founding a journalistic and political dynasty and, given how successful he’s been, Boris’s early life makes for fascinating reading. For instance, he only allowed his children to watch three television programmes – Vision On, Jackanory and Blue Peter – and Rachel was made to read out letters to the Times from the age of four. Indeed, Rachel learned to read before Boris in spite of being 15 months younger, a humiliation that Boris has described as “the single most galvanising event” of his life.

Yet alongside these intriguing titbits there’s a lot of guff about the “dark side” of Boris’s character that, frankly, doesn’t ring true. For instance, she claims that there’s a “Sicilian” side to the Johnson clan and tries to portray them as the Corleone’s of Notting Hill. “Cross them or criticise them – or worse still, mock them – at your peril,” she writes. “Accustomed to near-universal praise and affection, they ruthlessly close ranks against detractors.”

That isn’t my experience. When I was the drama critic of the Spectator, I joined forces with Lloyd Evans, another Spectator journalist, to write a bedroom farce called Who’s The Daddy? about Boris’s affair with Petronella Wyatt. Boris was the editor at the time and when the play became a smash hit he could have justifiably sacked us. Instead, he sent Lloyd and me a postcard. “I always knew my life would be turned into a farce,” he joked. “I’m just glad it’s been entrusted to two such distinguished men of letters.”

Lloyd and I mocked him again in 2009 when we co-wrote a dramadoc for Channel 4 called When Boris Met Dave that chronicled the rivalry between the Mayor of London and the Prime Minister that dates back to their time at Eton together. Even tolerant, happy-go-lucky politicians might have taken umbrage at this second offence, but not Boris. I bumped into him at the Spectator summer party shortly afterwards and he smiled ruefully and said if I could possibly refrain from writing a third farce about him he’d be eternally in my debt. Not exactly a horse’s head at the bottom of my bed.

The problem with this biography is that, at some fundamental level, Sonia Purnell just doesn’t “get” Boris. Most people are aware of Boris’s failings, but are willing to turn a blind eye because he adds to the gaiety of the nation. As Dominic Lawson, the ex-editor of the Sunday Telegraph puts it, “He exudes a general bonhomie and it’s contagious.”